20W50 is only slightly thicker when cold than a 15W50.
I wouldn't bother with any oil treatments unless they are ZDDP, Zinc Dithiophosphate, a anti wear anti scuff addition of around 0.012 % to oil quantity, was excellent esp for the older flat tappet type with high lift cams, and consequent higher loads, but is beneficial in any oil, most modern oils have boron based anti wear additives, some better than others.
It also provides oxidation stability, important when engines are town use or gentle use only, pampered engines dies early, usually as a result of the oil remaining too cold to burn off contaminants in it, mainly carbon,(that's why it goes black).
It is also highly beneficial in controlling lead/copper bearing corrosion, but because phosphorous is a poison as far as cats go the level has been reduced alarmingly, most of the newer oils are mostly ashless anti oxidants, much more cat friendly.
Oil is an amazingly interesting and complex issue.
If using your car hard then mineral oils will not be as stable as fully synthetics are, semi's are a waste of time imo.
tabetha
tabetha